Conventionally, an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction peripheral having functions of those apparatus may include a sheet feed device configured to feed a sheet by a feed roller, and a rotary developing device configured to develop an electrostatic latent image formed on a photosensitive drum. As such an image forming apparatus, there is an image forming apparatus in which the feed roller and the rotary developing device are driven by the drive of a drive motor being transmitted to the feed roller and the rotary developing device by a drive transmission apparatus at a predetermined timing.
FIG. 21 is a view illustrating a structure of a rotary developing device which is provided in a conventional electrophotographic color image forming apparatus and is configured to receive the drive transmitted by the drive transmission apparatus as described above. In FIG. 21, a rotary developing device 203A includes developing units 201Y, 201M, 201C, and 201K which contain yellow, magenta, cyan, and black developers, respectively. The developing units 201Y, 201M, 201C, and 201K include developing rollers 202Y, 202M, 202C, and 202K, respectively, and are supported by a rotary holder 203. Note that, FIG. 21 illustrates a state in which the yellow developing unit 201Y is located at a developing position, and in this state, the developing roller 202Y abuts against a photosensitive drum 204 to develop the latent image on the photosensitive drum 204 with the yellow developer.
A drive transmission apparatus 200 includes an input gear 205 which rotates at a constant velocity in the direction indicated by the arrow L in FIG. 21 by a drive source (not shown), and an output gear 206 which rotates at a predetermined timing. The output gear 206 includes a partially-toothless gear 206a having a toothless portion, and a control cylinder 206b to be latched by a solenoid 208. Further, one end of an urging spring 207 is hooked on the control cylinder 206b, and the urging spring 207 urges the output gear 206 in the direction indicated by the arrow H in FIG. 21.
Note that, in the drive transmission apparatus 200 having the above-mentioned structure, when the solenoid 208 is turned ON at a predetermined timing in the initial state in FIG. 21 so that the control cylinder 206b is unlatched, the urging spring 207 urges the output gear 206 to rotate in the direction indicated by the arrow H in FIG. 21. Accordingly, the partially-toothless gear 206a starts to mesh with the input gear 205 to transmit a drive force to the output gear 206. When the output gear 206 then performs substantially one revolution and the meshing of output gear 206 with the input gear 205 is terminated, the urging spring 207 urges the output gear 206 to rotate up to the latch position with the solenoid 208. At this time, when the solenoid 208 is turned OFF, the control cylinder 206b is latched by the solenoid 208, and the output gear 206 is brought into the initial state so that the drive force transmission is halted.
Further, in FIG. 21, a development switching mechanism 200A is configured to rotate the rotary holder 203 due to the drive force transmitted to the drive transmission apparatus 200 so as to switch the developing units 201Y, 201M, 201C, and 201K to be brought into abutment against the photosensitive drum 204. The development switching mechanism 200A includes a rotary holder gear 203a which is formed on an outer periphery of the rotary holder 203, and a small gear 209 which is provided coaxially with the output gear 206 and meshes with the rotary holder gear 203a. 
When the solenoid 208 is turned ON at the predetermined timing, the output gear 206 rotates in the direction indicated by the arrow H in FIG. 21, and the rotary holder 203 rotates in the direction indicated by the arrow W in FIG. 21. In this case, the number of teeth of the rotary holder gear 203a is “4n” (“n” is an integer) times larger than the number of teeth of the small gear 209, and hence, when the output gear 206 performs “n” revolutions by the drive transmission apparatus 200, the rotary holder 203 accurately performs a quarter revolution. Accordingly, the developing units 201Y, 201M, 201C, and 201K sequentially move to the developing position at predetermined timings to develop the latent images on the photosensitive drum 204 with the developers of the respective colors.
By the way, in the drive transmission apparatus 200 as described above, after the output gear 206 is unlatched from the solenoid 208, the urging spring 207 urges the output gear 206 with a force in its rotation direction (motive torque) so that the output gear 206 meshes with the input gear 205. On the other hand, an output member such as the rotary holder 203 is drivingly coupled to the output gear 206, and hence a drive load or inertial force of the output member is exerted on the output gear 206 in its reverse rotation direction.
Thus, a force which is equal to or greater than the drive load or inertial force of the output member is necessary as the motive torque, but the increase in motive torque results in upsizing of the solenoid 208 configured to latch and unlatch the output gear 206 in order to generate a force against the motive torque. Further, when the motive torque greater than necessary is exerted on the output gear 206, the output gear 206 unlatched from the solenoid 208 is rotated at high velocity, which leads to a risk of irregular rotation of the output gear 206.
Therefore, in the conventional technology, there is disclosed a structure in which a partially-toothless gear configured to mesh with an input gear 205 prior to an output gear 206 is provided so that the output gear meshes with the input gear in association with the rotation of the partially-toothless gear (see Patent Literature 1). With this structure, the motive torque for the partially-toothless gear can be set irrespective of a drive load or inertial force of an output member coupled to the output gear, and hence the problem arising from the motive torque is solved to some extent.
In recent years, higher-speed image forming apparatus have been developed, and on the other hand, ecological measures have been demanded on the apparatus main body. In particular, there has been demanded suppression of noise of the image forming apparatus in operation. In the rotary holder rotation control to be performed using the conventional drive transmission apparatus 200 as illustrated in FIG. 21, however, in the initial stage and the later stage of the rotary holder rotation, the developing roller is separated from and brought into abutment against the photosensitive drum at the developing position. Therefore, along with the increase in operation speed of the image forming apparatus, a period required to switch the developing units of the respective colors is shortened, in other words, an angular velocity of the rotary holder is increased. As a result, vibration and impact sound of the image forming apparatus at the time of separation and abutment become conspicuous.
In view of the above, in order to suppress the impact sound, for example, there is a technology of reducing the angular velocity of the rotary holder only at the time of separation and abutment. FIG. 22 is a view illustrating a structure configured to set a variable angular velocity at the time of drive transmission. In FIG. 22, an input gear 1211 includes an input small gear 1211a and an input large gear 1211b, and rotates at a constant velocity in the direction indicated by the arrow R in FIG. 22. An output gear 1212 receives the drive transmitted from the input gear 1211. The output gear 1212 includes an output large gear 1212a and an output small gear 1212b which have toothless portions and mesh with the input small gear 1211a and the input large gear 1211b of the input gear 1211 in an alternate manner.
In a state in which the input small gear 1211a meshes with the output large gear 1212a, the output gear 1212 is driven at a low velocity, and in a state in which the input large gear 1211b meshes with the output small gear 1212b, the output gear 1212 is driven at a high velocity. Accordingly, the output gear 1212 can be rotated at the low velocity only at the time of abutment and separation, and thus the impact sound can be suppressed.